In mobile communications, there is an increasing demand for higher system capacity and end-user data rates. For this purpose, communication systems are densified more and more by providing a higher number of access nodes with smaller distances (from one access node to another access node) as compared with common communication systems. Still further, demands for very high system capacity and very high end-user date rates can be met by so-called Ultra-Dense Networks (UDNs). UDNs may be regarded as networks with access-node densities considerably higher than the densest cellular networks of today. Such UDNs may be set up with distances between access nodes from a few meters in indoor deployments up to around 50 m in outdoor deployment.
Data rates of the order of 10 Gigabits per second (Gbps) can be practically achieved only with a sufficiently large transmission bandwidth, significantly larger than the current maximum of 100 MHz for the Long Term Evolution (LTE) standard. UDNs may be expected to use a maximum transmission bandwidth of up to around 1 to 2 GHz. Such very wide transmission bandwidths are realistically only possible at higher frequency bands beyond 10 GHz. For example, frequencies in the lower part of the millimeter wave band up to 100 GHz may be of specific interest for UDNs.
In situations where directive beamforming is used in a UDN, informing other access nodes of upcoming use of communication resources, or spreading information on resource reservations, in a distributed fashion, is non-trivial. In the UDN context wireless self-backhaul for a set of UDN nodes and interference aware routing solutions for routing packets through the backhaul networks have been proposed by D. Hui and J. Axnäs in the paper “Joint Routing and Resource Allocation for Wireless Self-Backhaul in an Indoor Ultra-Dense Network”, PIMRC 2013. With self-backhauling, an access node serves not only its own assigned UEs in the vicinity but also its neighbouring access nodes as a relaying node in order to route data towards and/or from an information aggregation node. To maximize the throughput of each route, a route selection process takes into account the mutual interference among wireless links. According the concept of the aforementioned paper, one approach is to jointly optimize route selection and radio resource allocation. For this purpose, the original network may be transformed to an expanded virtual network in which each virtual node represents a possible way of allocating radio resources to the access node. A route selected in such a virtual network jointly determines a sequence of access nodes (i.e. the real route) and the corresponding radio resources allocated to the links associated with these nodes. This and similar concepts provide a solution focusing on interference aware routing under full buffer assumptions.
The Wi-Fi family IEEE 802.11 uses most commonly a distributed coordination function (DCF) based on users contending for the resources. Each user backs off a random time interval before accessing the channels. This procedure ensures long term fairness in the access of the communication resources, but inherently relies on overhearing other nodes transmissions and hence omni-directional transmissions. Procedures that work fine for omni-directional transmissions, e.g. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) procedures, may be inefficient and/or not work in an UDN environment due to transmit and receive beamforming.
The mesh coordination function controlled channel access (MCCA) in IEEE 802.11s specifies that the stations/nodes with this functionality enabled may make reservations of the wireless medium. The reservations are typically for omni-directional transmissions, but not necessarily so. The procedure is as follows:    1) A first station transmits a MCCA Setup Request. This station becomes a mesh coordination function controlled channel access opportunity (MCCAOP) owner. The transmission is omni-directional and the destination address of the message may be a single station or group destination address. The MCCA Setup Request specifies channel reservations for the entire channel with a specified start time, duration, and periodicity of the transmission. Each channel reservation has a reservation ID number, to uniquely identify the reservation.    2) The receivers of the reservation, the MCCAOP responders, return a MCCA Setup Reply to accept or reject the reservation. If the reservation is rejected the reply may contain a suggestion of a different reservation for the first station to use.
The 802.11s standard is designed to be operating in an environment where mobile stations transmit using omni-directive transmissions. For this reason the standard does not include the intended receiver of the transmission that will take place during the MCCA Setup Request, i.e., the resource reservation, or any other reservation message. Hence the standard does not provide for easy spatial reuse of the transmission resources in environments where directive transmissions are employed and/or required.
The MCCA relies on omni-directional transmissions. Directional transmissions increase the hidden node probability and hence the receivers of the MCCA Setup Requests have a larger probability to be hidden nodes (from the perspective of the MCCAOP owner), i.e., they do not overhear the MCCA Setup Request because it is sent in another direction. Hence in these environments there is a high probability that the MCCA procedure will generate conflicting reservations (when the hidden nodes try to reserve the same resources as sending the initial MCCA Setup Request that these nodes did not receive). Such conflicting reservations would cause degraded network performance (throughput) by increased interference and a high number of colliding transmissions.
In addition, the MCCA function allocates the entire channel for a time duration, which is suboptimal when considering very wide channels, e.g., of the order 1-2 GHz which is expected to be used by UDNs. Furthermore, in the MCCA function there is no support for traffic classes with different priorities.